devserver: Fix linter errors

Also remove devserver_exception.py as there was no user except devserver
itself.

BUG=chromium:1003986
TEST=./devserver_integration_test.py

Change-Id: Idf5289371bbe17c57fbbdb5de445e083281bd62c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/1838133
Tested-by: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
4 files changed